1. Field of the Invention
This invention relates to a control system for controlling the operation of an internal combustion engine, and more particularly to a control system which is adapted to change a control mode depending on an exhaust mode which determines a path through which exhaust gases are exhausted.
2. Prior Art
A control system for controlling the operation of an internal combustion engine is conventionally Known, which has two (first and second) catalytic converters arranged in series in an exhaust passage of an internal combustion engine. The first catalytic converter has a capacity smaller than that of the second converter and is arranged at a location relatively close to the engine, whereby it is possible to effect efficient purification of exhaust gases emitted from the engine when it is started at a low temperature, through accelerated activation of the catalytic converter. However, according to the control system, the first catalytic converter arranged in the exhaust passage at a location relatively close to the engine is exposed to exhaust gases at a high temperature during engine operation after the engine has been warmed up, so that the first catalytic converter is deteriorated at a higher speed, resulting in a shortened service life thereof.
To overcome such an inconvenience, an exhaust gas purifying system has already been proposed in Japanese Provisional Utility Model Registration Publication (Kokai) No. 52-135713, which has a bypass passage bypassing the first catalytic converter, and a selector valve for changing over the flow path of exhaust gases between one passing through the first catalytic converter and one passing through the bypass passage.
According to this prior art, when the engine is started at a low temperature, exhaust gases can be purified efficiently by the first catalytic converter, and then the selector valve is operated to select the flow path of exhaust gases passing through the bypass passage after the engine has been warmed up, whereby the second catalytic converter alone is operative to purify the exhaust gases, to thereby make it possible to prolong the service life of the first catalytic converter.
However, when the selector valve is operated to select the flow path of exhaust gases passing through the first catalytic converter, the pressure of exhaust gases increases to lower the exhaust efficiency of the engine, so that the intake efficiency .eta.V is lowered, whereas when the selector valve is operated to select the flow path passing through the bypass passage, the pressure of the exhaust gases decreases to increase the exhaust efficiency, so that the intake efficiency .eta.V is enhanced. In spite of such a variation in the intake efficiency .eta.V, the fuel supply and ignition timing are not changed correspondingly to changeover of the flow path of exhaust gases, which prevents the air-fuel ratio and the state of combustion from being stabilized, resulting in degraded exhaust emission characteristics.
On the other hand, an air-fuel ratio control system has been proposed e.g. by Japanese Provisional Patent Publication (Kokai) No. 2-67443, which comprises a linear output air-fuel ratio sensor (hereinafter referred to as "the LAF sensor") having an output characteristic which is substantially proportional to the concentration of oxygen present in exhaust gases, arranged in an exhaust passage of an engine at a location upstream of a catalytic converter, and an O2 sensor arranged in the exhaust passage at a location downstream of the catalytic converter, an output from the O2 sensor which drastically changes when the air-fuel ratio of a mixture supplied to the engine changes across the stoichiometric air-fuel ratio. According to the proposed system, the desired output voltage of the LAF sensor or desired air-fuel ratio coefficient is corrected based on the output from the O2 sensor, and the output from the LAF sensor is feedback-controlled to the corrected desired output voltage or an equivalent ratio of the output from the LAF sensor is feedback-controlled to the corrected desired air-fuel ratio coefficient, to control the air-fuel ratio to the stoichiometric air-fuel ratio.
If the aforementioned selector valve and the bypass passage are additionally provided in this proposed air-fuel ratio control system having two oxygen sensors (the LAF sensor and the O2 sensor), presumably it is possible to constantly control the air-fuel ratio to the stoichiometric air-fuel ratio irrespective of whether the engine has been warmed up or not, and hence to further improve exhaust emission characteristics.
However, in this combination, when the selector valve is operated to select the flow path of exhaust gases through the bypass passage, the oxygen storage effect of the first catalytic converter is not obtained, so that the output from the O2 sensor downstream of the first catalytic converter directly reflects the oxygen concentration of exhaust gases emitted from the combustion chambers of the engine so that the repetition period of inversion of the sensor output between the rich side and the lean side becomes short against its originally intended purpose. Therefore, if the desired air-fuel ratio coefficient is corrected based on the output from the O2 sensor downstream of the first catalytic converter to feedback-control the output from the LAF sensor to the corrected desired air-fuel ratio coefficient, when exhaust gases are guided through the bypass passage, the degree of convergence of the air-fuel ratio to the desired value is liable to be lower, leading to degraded exhaust emission characteristics.